As others have mentioned, you will be able to board outside of the time frame they gave you. They were strict about that at one point in time but it was total chaos and some passengers complained.
If the ship is not quite ready for boarding, you will be asked to take a seat and will be given a number for boarding. The BVE passengers and any special groups (such as wedding parties) will be allowed to board first, followed by the suite passengers, elite passengers and then general boarding by number. But, that is only if you are required to wait.
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Cruise Raider